The existence of Novarossi .28 models w/o a pull starter proves it is possible. In the past I have made a .28 backplate.

Basically all Novarossi crankcases and so all backplates are the same, If you have a crank case with a 25.8 bearing you can get the backplate of a .21 engine, 2 things you have to do. enlarge the flat spot that makes space for the piston because the larger diameter of it and you have to grind off the pull start pin from the crankshaft